Multi-Stack Package-on-Package Structures

ABSTRACT

A package includes a first device die, and a first encapsulating material encapsulating the first device die therein. A bottom surface of the first device die is coplanar with a bottom surface of the first encapsulating material. First dielectric layers are underlying the first device die. First redistribution lines are in the first dielectric layers and electrically coupling to the first device die. Second dielectric layers are overlying the first device die. Second redistribution lines are in the second dielectric layers and electrically coupling to the first redistribution lines. A second device die is overlying and electrically coupling to the second redistribution lines. No solder region connects the second device die to the second redistribution lines. A second encapsulating material encapsulates the second device die therein. A third device die is electrically coupled to the second redistribution lines. A third encapsulating material encapsulates the third device die therein.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.14/972,622, entitled “Multi-Stack Package-on-Package Structures,” filedon Dec. 17, 2015, which application claims the benefit of the followingprovisionally filed U.S. Patent application: Application Ser. No.62/253,401, filed Nov. 10, 2015, and entitled “Multi-Stack Package onPackage-on-Package Structures,” which applications are herebyincorporated herein by reference.

BACKGROUND

In a conventional Integrated Fan-Out (InFO) process, a top package, inwhich a first device die is bonded, is bonded to a bottom package. Thebottom package may also have a device die packaged therein. By adoptingthe InFO process, the integration level of the packages is increased.

In an existing InFo process, the bottom package is formed first, whichincludes encapsulating a molding compound on a device die and aplurality of through-molding vias. Redistribution lines are formed toconnect to the device die and the through-molding vias. A top package,which may include device dies bonded to an additional package substrate,is then bonded to the bottom package through solder joints.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 11A illustrate the cross-sectional views of intermediatestages in the formation of a package including multi-stack dies inaccordance with some embodiments.

FIGS. 11B through 16 illustrate the cross-sectional views of packagesincluding multi-stack dies in accordance with some embodiments.

FIG. 17 illustrates a process flow for forming a package in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A multi-stack package and the method of forming the package are providedin accordance with various exemplary embodiments. Some variations ofsome embodiments are discussed. Throughout the various views andillustrative embodiments, like reference numbers are used to designatelike elements. Throughout the description, the term “multi-stackpackage” refers to a package in which two or more levels of device dies,each encapsulated in an encapsulating material, have no solder regionstherebetween. Furthermore, throughout the description, the surfaces ofdevice dies having metal pillars are referred to as the front surfacesof the respective device dies, and the surfaces opposite to the frontsurfaces are back surfaces. The back surfaces are also the surfaces ofsemiconductor substrates of the respective device dies in accordancewith some embodiments.

FIGS. 1 through 11A illustrate the cross-sectional views of intermediatestages in the formation of a package in accordance with someembodiments. In the subsequent discussion, the process steps shown inFIGS. 1 through 11A are discussed referring to the process flow 600shown in FIG. 17.

FIGS. 1 and 2 illustrate the formation of through-vias 32. Therespective step is shown as step 602 in the process flow shown in FIG.17. Referring to FIG. 1, carrier 20 is provided, and adhesive layer 22is disposed over carrier 20. Carrier 20 may be a blank glass carrier, ablank ceramic carrier, or the like, and may have a shape of asemiconductor wafer with a round top-view shape. Carrier 20 is sometimesreferred to as a carrier wafer. Adhesive layer 22 may be formed of aLight-to-Heat Conversion (LTHC) material, for example, although othertypes of adhesives may be used. In accordance with some embodiments ofthe present disclosure, adhesive layer 22 is capable of decomposingunder the heat of light, and hence can release carrier 20 from thestructure formed thereon.

Also referring to FIG. 1, dielectric layer 24 is formed over adhesivelayer 22. In accordance with some embodiments of the present disclosure,dielectric layer 24 is a polymer layer formed of a polymer, which may bea photo-sensitive polymer such as polybenzoxazole (PBO), polyimide, orthe like. In accordance with some embodiments, dielectric layer 24 isformed of a nitride such as silicon nitride, an oxide such as siliconoxide, PhosphoSilicate Glass (PSG), BoroSilicate Glass (BSG),Boron-doped PhosphoSilicate Glass (BPSG), or the like.

Conductive seed layer 26 is formed over dielectric layer 24, forexample, through Physical Vapor Deposition (PVD). Conductive seed layer26 may be a metal seed layer including copper, aluminum, titanium,alloys thereof, or multi-layers thereof. In accordance with someembodiments of the present disclosure, conductive seed layer 26 includesa first metal layer such as a titanium layer (not shown) and a secondmetal layer such as a copper layer (not shown) over the first metallayer. In accordance with alternative embodiments of the presentdisclosure, conductive seed layer 26 includes a single metal layer suchas a copper layer, which may be formed of substantially pure copper or acopper alloy.

As shown in FIG. 1, mask layer 28 (such as a photo resist) is appliedover conductive seed layer 26, and is then patterned using a photolithography mask. In accordance with some embodiments of the presentdisclosure, mask layer 28 is formed of a dry film, which is laminatedonto conductive seed layer 26. In accordance with some embodiments, masklayer 28 is formed of a photo resist, which is applied by spin coating.As a result of the patterning (exposure and development), openings 30are formed in mask layer 28, through which some portions of conductiveseed layer 26 are exposed.

Through-vias 32 are formed in openings 30 through plating, which may beelectro-plating or electro-less plating. Through-vias 32 are plated onthe exposed portions of conductive seed layer 26. Through-vias 32 areconductive, and may be metal vias including copper, aluminum, tungsten,nickel, or alloys thereof. The top-view shapes of through-vias 32include, and are not limited to, rectangles, squares, circles, and thelike. The heights of through-vias 32 are determined by the thickness ofthe subsequently placed device dies 34 (FIG. 3), with the heights ofthrough-vias 32 slightly greater than or equal to the thickness ofdevice dies 34 in accordance with some embodiments of the presentdisclosure.

After the plating of through-vias 32, mask layer 28 is removed. As aresult, the portions of conductive seed layer 26 that are previouslycovered by mask layer 28 are exposed. Next, an etching step is performedto remove the exposed portions of conductive seed layer 26, wherein theetching may be an anisotropic or isotropic etching. The portions ofconductive seed layer 26 (FIG. 1) that are overlapped by through-vias32, on the other hand, remain not etched. The resulting through-vias 32are shown in FIG. 2. Throughout the description, the remainingunderlying portions of conductive seed layer 26 are referred to as thebottom portions of through-vias 32, and are not shown separately.Conductive seed layer 26 and the overlying portions of through-vias 32may or may not have distinguishable interfaces. For example, the copperlayer in conductive seed layer 26 may be merged with through-vias 32with no distinguishable interfaces. The titanium layer in conductiveseed layer 26 may be distinguishable from the copper-containingthrough-vias 32. As a result of the etching of conductive seed layer 26,dielectric layer 24 is exposed.

FIG. 3 illustrates the placement of device dies 34 over dielectric layer24. The respective step is shown as step 604 in the process flow shownin FIG. 17. Device dies 34 may be adhered to dielectric layer 24 throughdie-attach films 38, which are adhesive films. The edges of die-attachfilms 38 are co-terminus with (aligned to) the respective edges ofdevice dies 34. Device dies 34 may include semiconductor substrates 36having back surfaces (the surfaces facing down) in physical contact withthe respective underlying die-attach films 38. Device dies 34 furtherincludes integrated circuit devices 40 (such as active devices orpassive devices) at the front surfaces (the surfaces facing up) of therespective semiconductor substrates 36. Device dies 34 may be memorydies such as Static Random Access Memory (SRAM) dies, Dynamic RandomAccess Memory (DRAM) dies, flash memory dies, etc. Device dies 34 may beidentical to each other.

In accordance with some embodiments, device dies 34 do not havethrough-vias in semiconductor substrates 36. In accordance withalternative embodiments, device dies 34 have through-vias 42 extendinginto semiconductor substrates 36. In the embodiments in which there arethrough-vias 42, through-vias 32 may not (or may) be formed sincethrough-vias 42 may act as the electrical connections forinterconnecting the conductive features overlying and underlying devicedies 34. Accordingly, through-vias 32 do not have to be formed, and therespective manufacturing cost may be saved. In accordance with someembodiments, some or all of through-vias 42 are solely for electricallyinterconnecting the conductive features overlying and underlying devicedies 34, and are not electrically connected/coupled to any active andpassive devices 40 such as transistors, diodes, capacitors, resistors,etc. Throughout the description, when features (such as through-vias 32and 42) are shown as dashed, it is indicated that these features may ormay not be formed.

Device dies 34 may include metal pillars 44 close to their top surfaces.Metal pillars 44 are electrically coupled to integrated circuits 40inside device dies 34. In accordance with some exemplary embodiments ofthe present disclosure, metal pillars 44 are covered by dielectriclayers 46, with the top surfaces of dielectric layers 46 being higherthan the top surfaces of metal pillars 44. Dielectric layers 46 furtherextend into the gaps between metal pillars 44. In accordance withalternative embodiments of the present disclosure, the top surfaces ofmetal pillars 44 are coplanar with the top surfaces of the respectivedielectric layers 46. Dielectric layers 46 may be formed of a polymersuch as polybenzoxazole (PBO) or polyimide in accordance with someexemplary embodiments. Metal pillars 44 may be copper pillars, and mayalso include other conductive/metallic materials such as aluminum,nickel, or the like.

Referring to FIG. 4, encapsulating material 48 is encapsulated on devicedies 34 and through-vias 32. The respective step is shown as step 606 inthe process flow shown in FIG. 17. Encapsulating material 48 fills thegaps between neighboring device dies 34, and encircles each of devicedies 34 and through-vias 32. Encapsulating material 48 may include amolding compound, a molding underfill, an epoxy, and/or a resin. Afterthe encapsulating process, the top surface of encapsulating material 48is higher than the top surfaces of metal pillars 44 and through-vias 32.

Next, a planarization step such as a Chemical Mechanical Polish (CMP)step or a grinding step is performed to planarize encapsulating material48, until through-vias 32 are exposed. Metal pillars 44 of device dies34 are also exposed as a result of the planarization. Due to theplanarization, the top surfaces of through-vias 32 are substantiallylevel (coplanar) with the top surfaces of metal pillars 44, and aresubstantially level (coplanar) with the top surface of encapsulatingmaterial 48.

Referring to FIG. 5, a plurality of dielectric layers 50 and therespective Redistribution Lines (RDLs) 52 are formed over encapsulatingmaterial 48, through-vias 32, and metal pillars 44. The respective stepis shown as step 608 in the process flow shown in FIG. 17. In accordancewith some embodiments of the present disclosure, dielectric layers 50are formed of a polymer(s) such as PBO, polyimide, or the like. Inaccordance with alternative embodiments of the present disclosure,dielectric layers 50 are formed of an inorganic dielectric material(s)such as silicon nitride, silicon oxide, silicon oxynitride, or the like.

RDLs 52 are electrically coupled to metal pillars 44 and through-vias32, and may interconnect metal pillars 44 and through-vias 32 with eachother. RDLs 52 may include metal traces (metal lines) and viasunderlying and connected to the metal traces. In accordance with someembodiments of the present disclosure, RDLs 52 are formed throughplating processes, wherein each of RDLs 52 includes a seed layer (notshown) and a plated metallic material over the seed layer. The seedlayer and the plated metallic material may be formed of the samematerial or different materials.

Referring to FIG. 5, dielectric layers 50 include a top dielectric layeroverlying RDLs 52, wherein some metal pads of RDLs 52 are exposedthrough openings 54 in the top dielectric layer 50.

Next, Referring to FIG. 6, through-vias 56 are formed over dielectriclayers 50 and RDLs 52. The respective step is shown as step 610 in theprocess flow shown in FIG. 17. The formation process may include forminga seed layer (not shown) over dielectric layers 50 and extending intoopenings 54 (FIG. 5), forming a patterned mask layer (not shown), withopenings 54 exposed to the openings in the patterned mask layer, platingthrough-vias 56 in the openings in the patterned mask layer, removingthe patterned mask layer, and etching the seed layer.

The seed layer of through-vias 56 may include a titanium layer and acopper layer over the titanium layer. The plated material may have auniform composition and may be formed of copper or a copper alloy. Theplated material includes some portions over the top surface of the topdielectric layer 50, and other portions extending into openings 54 (FIG.5).

FIG. 6 also illustrates the adhesion of device die 58 onto dielectriclayers 50 through, for example, die-attach film 60. The respective stepis shown as step 612 in the process flow shown in FIG. 17. The backsurface of device die 58, which back surface may be the back surface ofthe semiconductor substrate in device die 58, is in contact withdie-attach film 60. Device die 58 may be a logic die such as a CentralProcessing Unit (CPU) die, a Graphic Processing Unit (GPU) die, or thelike. Device die 58 includes metal pillars 62 in surface dielectriclayer 64. Surface dielectric layer 64 may be formed of PBO, for example,or other dielectric materials.

FIG. 7 illustrates the encapsulation of through-vias 56 and device dies58 with encapsulating material 66. The respective step is shown as step614 in the process flow shown in FIG. 17. Encapsulating material 66 maybe a molding compound. After the dispensing and the curing of theencapsulating material 66, a planarization is performed to remove excessencapsulating material 66, so that through-vias 56 and metal pillars 62are exposed.

Next, referring to FIG. 8, dielectric layers 68 and RDLs 70 are formedover encapsulating material 66 and device dies 58. The respective stepis shown as step 616 in the process flow shown in FIG. 17. Dielectriclayers 68 may also be formed of polymers such as PBO or polyimide. RDLs70 are electrically coupled to through-vias 56 and metal pillars 62.Furthermore, RDLs 70 may also electrically connect through-vias 56 tometal pillars 62.

Further referring to FIG. 8, Under-Bump-Metallurgies (UBMs) 72 andelectrical connectors 74 are formed in accordance with some exemplaryembodiments of the present disclosure. Electrical connectors 74 areelectrically coupled to RDLs 70 and 52, metal pillars 62 and 44, and/orthrough-vias 32, 42, and 56. The formation of electrical connectors 74may include placing solder balls over RDLs 70, and then reflowing thesolder balls. In accordance with alternative embodiments of the presentdisclosure, the formation of electrical connectors 74 includesperforming a plating process to form solder regions over RDLs 70 andthen reflowing the solder regions. Electrical connectors 74 may alsoinclude metal pillars, or metal pillars and solder caps, which may alsobe formed through plating.

Throughout the description, the structure overlying adhesive layer 22 isreferred to as wafer-level package 76, which may be a composite wafer.Next, package 76 is de-bonded from carrier 20. In accordance with someexemplary de-bonding processes, as shown in FIG. 9, carrier 78 isattached to package 76 to protect electrical connectors 74. Therespective step is shown as step 618 in the process flow shown in FIG.17. Carrier 78 may be a dicing tape fixed onto a dicing frame (notshown). The de-bonding is performed, for example, by projecting a UVlight or a laser on adhesive layer 22 (FIG. 8). For example, whenadhesive layer 22 is formed of LTHC, the heat generated from the lightor laser causes the LTHC to be decomposed, and hence carrier 20 isdetached from wafer-level package 76. The resulting structure is shownin FIG. 9.

FIG. 10 illustrates the patterning for forming openings 80 in dielectriclayer 24. The respective step is shown as step 620 in the process flowshown in FIG. 17. For example, when dielectric layer 24 is a polymerlayer, it can be patterned using laser drill to remove the portionsoverlapping through-vias 32, so that through-vias 32 are exposed throughopenings 80.

FIG. 11A illustrates the bonding of package 200 to package 76, thusforming PoP package 82. The respective step is shown as step 622 in theprocess flow shown in FIG. 17. Packages 76 and 200 are also referred toas a primary package and a secondary package, respectively. The bondingis performed through solder regions 84, which join through-vias 32 tothe metal pads in the overlying package 200. In accordance with someembodiments of the present disclosure, package 200 includes devicedie(s) 234, which may be memory dies such as SRAM dies, DRAM dies, orthe like. Device dies 234 may also be identical to device dies 34. Thememory dies are also bonded to package substrate 202 in accordance withsome exemplary embodiments. Encapsulating material 90 encapsulatesdevice dies 234 therein, wherein encapsulating material 90 may be amolding compound, a molding underfill, etc. After the bonding ofsecondary package 200 to primary package 76, underfill 86 is dispensedinto the gap between secondary package 200 and primary package 76, andis then cured. A die-saw may then be performed to saw package 82 intoindividual packages 88, which are identical to each other. Therespective step is shown as step 624 in the process flow shown in FIG.17.

As a result of the die-saw, the respective edges of encapsulatingmaterial 48, encapsulating material 66, dielectric layers 50, anddielectric layers 68 are aligned with each other. The edges ofencapsulating material 90 and package substrate 202 may or may not bealigned to the edges of underlying package 76.

In accordance with some embodiments in which through-vias 42 are formed,after the structure as shown in FIG. 9 is formed, a backside grinding isperformed to remove die-attach films 38 and some portions ofsemiconductor substrates 36, until through-vias 42 are exposed. Next, asshown in FIG. 11B, RDLs 43 are formed over and electrically coupled tothrough-vias 42. Through-vias 32 may or may not be formed in accordancewith some embodiments when through-vias 42 are formed. In accordancewith some embodiments, through-vias 42 act as the interconnectionbetween RDLs 43 and RDLs 52 (through metal lines and vias (not shown)between through-vias 42 and metal pillars 44). Through-vias 42 may beused solely for the interconnection of RDLs 43 and 52, and are notelectrically coupled to any passive or active device in device dies 34.This has the advantageous feature of increasing the total number ofthrough-vias since through-vias 42 may be formed smaller thanthrough-vias 32. In addition, the cost that otherwise will be incurredfor forming through-vias 32 is saved.

In the package shown in FIGS. 11A and 11B, device dies 58, 34, and 234form a multi-stack package including two encapsulating regions/materialsseparated by intermediate dielectric layers 50 and RDLs 52. Stackingdevice dies 34 over the encapsulated device die 58 results in a verythin package since no solder joints are used between device dies 34 anddevice die 58. In addition, two or more device dies 34 may be located inthe same encapsulating material 48, and hence the height of package 88is further reduced. The footprint (top-view area) of package 88,however, is not increased since device die 58 has a larger top-view areathan device dies 34.

FIGS. 12 through 16 illustrate packages 88 in accordance with someembodiments of the present disclosure. Unless specified otherwise, thematerials and the formation methods of the components in theseembodiments are essentially the same as the like components, which aredenoted by like reference numerals in the embodiments shown in FIGS. 1through 11A and 11B. The details regarding the formation processes andthe materials of the components shown in FIGS. 12 through 16 may thus befound in the discussion of the embodiments shown in FIGS. 1 through 11Aand 11B. In each of these embodiments, device dies 234 may be identicalto or different from device dies 34. Furthermore, in the embodimentsshown in each of FIGS. 12 through 16, either through-vias 32 orthrough-vias 42 are formed, or both through-vias 32 and through-vias 42are formed.

FIG. 12 illustrates package 88 that has no solder regions betweendifferent levels of device dies 58, 34, and 234. Device dies 234 areelectrically coupled to device dies 34 through RDLs 92, which are formedin dielectric layers 94. In accordance with some embodiments of thepresent disclosure, the formation process of the structure in FIG. 12may include encapsulating device dies 234 in encapsulating material 90,followed by forming RDLs 92 and dielectric layers 94. The subsequentsteps are essentially shown in FIGS. 1 through 8. By using theembodiments in FIG. 12, the thickness of the resulting package 88 isfurther reduced since there is no solder region in the resultingpackage.

FIG. 13 illustrates package 88 in accordance with some embodiments.These embodiments are similar to the embodiments in FIGS. 11A and 11B,except primary package 76 has a single level of device die 58, whilesecondary package 200 includes multi-stacked device dies 34 and 234.

FIG. 14 illustrates package 88 in accordance with some embodiments.These embodiments are similar to the embodiments in FIGS. 11A and 11B,except both primary package 76 and secondary package 200 includemulti-stacked device dies. For example, primary package 76 includesdevice die 58 and device dies 34 forming a multi-stack package.Secondary package 200 includes device die 234 and device dies 334forming a multi-stack package. Device dies 234 may be identical todevice dies 334 or different from device dies 334. Device die 334 arefurther encapsulated in encapsulating material 348.

FIG. 15 illustrates package 88 in accordance with some embodiments.These embodiments are similar to the embodiments in FIGS. 11A and 11B,except secondary package 200 includes device dies 434 that are bonded tothe respective package substrate 202 through wire bonds. Device dies 34may be different from device dies 234. For example, device dies 34 maybe DRAM dies, while device dies 234 may be flash memory dies.

FIG. 16 illustrates package 88 in accordance with some embodiments.These embodiments are similar to the embodiments in FIG. 15, except thedevice dies 34 in FIG. 15 are replaced by die stacks 34′, with each ofthe die stacks 34′ including a plurality of device dies 534 bondedtogether. Die stacks 34′ are pre-formed before being used to formpackage 88. Device dies 534 in die stacks 34′ are bonded by solderregions 536. Furthermore, device dies 534 include through-vias 538penetrating through the respective semiconductor substrates.

The embodiments of the present disclosure have some advantageousfeatures. By forming multi-stack packages, the solder regions that areused in conventional Package-on-Package (PoP) structures are eithereliminated or at least reduced in number. Accordingly, the thickness ofthe resulting package is reduced.

In accordance with some embodiments of the present disclosure, a packageincludes a first device die, and a first encapsulating materialencapsulating the first device die therein. A bottom surface of thefirst device die is coplanar with a bottom surface of the firstencapsulating material. First dielectric layers are underlying the firstdevice die. First redistribution lines are in the first dielectriclayers and electrically coupling to the first device die. Seconddielectric layers are overlying the first device die. Secondredistribution lines are in the second dielectric layers andelectrically coupling to the first redistribution lines. A second devicedie is overlying and electrically coupling to the second redistributionlines. No solder region connects the second device die to the secondredistribution lines. A second encapsulating material encapsulates thesecond device die therein. A third device die is electrically coupled tothe second redistribution lines. A third encapsulating materialencapsulates the third device die therein.

In accordance with some embodiments of the present disclosure, a methodincludes placing a first device die over a carrier, encapsulating thefirst device die in a first encapsulating material, performing a firstplanarization to reveal first metal pillars in the first device die,forming first dielectric layers over the first device die and the firstencapsulating material, and forming first redistribution lines in thefirst dielectric layers. The first redistribution lines are electricallycoupled to the first metal pillars. The method further includes adheringa second device die to a top surface of the first dielectric layers,forming a first through-via over the first dielectric layers,encapsulating the second device die and the first through-via in asecond encapsulating material, performing a second planarization toreveal the first through-via and second metal pillars in the seconddevice die, forming second dielectric layers over the second device die,and forming second redistribution lines in the second dielectric layers.The second redistribution lines are electrically coupled to the secondmetal pillars and the first through-via.

In accordance with some embodiments of the present disclosure, a methodincludes placing a first device die over a carrier. The first device dieincludes a first semiconductor substrate, and first through-viaspenetrating through the first semiconductor substrate. The methodfurther includes encapsulating the first device die in a firstencapsulating material, forming first dielectric layers over the firstdevice die, and forming first redistribution lines in the firstdielectric layers. The first redistribution lines are electricallycoupled to first metal pillars in the first device die. A second devicedie is adhered to a top surface of the first dielectric layers. Secondthrough-vias are formed over the first dielectric layers. The secondthrough-vias are electrically coupled to the first redistribution lines.The method further includes encapsulating the second device die in asecond encapsulating material, forming second dielectric layers over thesecond device die, and forming second redistribution lines in the seconddielectric layers. The second redistribution lines are electricallycoupled to second metal pillars in the second device die. The carrier isde-bonded from the first device die. A backside grinding is performed onthe semiconductor substrate to reveal the first through-vias. Thirdredistribution lines are formed to electrically couple to the firstthrough-vias.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a first device die; a firstencapsulating material encapsulating the first device die therein; firstdielectric layers underlying the first device die, wherein both thefirst encapsulating material and the first device die are in contactwith a top layer in the first dielectric layers; first redistributionlines in the first dielectric layers and electrically coupling to thefirst device die; second dielectric layers overlying the first devicedie; second redistribution lines in the second dielectric layers andelectrically coupling to the first redistribution lines; a second devicedie overlying and electrically coupling to the second redistributionlines, wherein no solder region connects the second device die to thesecond redistribution lines; a second encapsulating materialencapsulating the second device die therein; a third device dieelectrically coupling to the second redistribution lines; and a thirdencapsulating material encapsulating the third device die therein. 2.The package of claim 1, wherein the third device die is overlying thesecond device die, and no solder region is between the third device dieand the first redistribution lines.
 3. The package of claim 1, whereinthe third device die is overlying the second device die, and the packagefurther comprises solder regions electrically coupling the third devicedie to the second redistribution lines.
 4. The package of claim 1further comprising a through-via penetrating through the secondencapsulating material, wherein the through-via electrically couples thethird device die to the second redistribution lines.
 5. The package ofclaim 1, wherein the second device die comprises: a semiconductorsubstrate; and through-semiconductor vias penetrating through thesemiconductor substrate, wherein the through-vias electrically couplethe second redistribution lines to the third device die.
 6. The packageof claim 5, wherein the through-semiconductor vias are electricallydecoupled from all active and passive devices in the second device die.7. The package of claim 6, wherein no through-via penetrates through thesecond encapsulating material.
 8. The package of claim 1, whereinrespective edges of the first dielectric layers, the second dielectriclayers, the first encapsulating material, and the second encapsulatingmaterial are aligned with each other.
 9. The package of claim 1 furthercomprising: a die-attach film between and physically contacting thefirst device die and the second dielectric layers, wherein both thefirst device die and the second device die have front surfaces facingthe first dielectric layers.
 10. A package comprising: a first pluralityof dielectric layers, wherein the first plurality of dielectric layershas a bottom surface and a top surface; a first through-via underlyingand in contact with the bottom surface of the first plurality ofdielectric layers; a second through-via overlying and in contact withthe top surface of the first plurality of dielectric layers; a firstdevice die and a second device die; a first encapsulating materialencapsulating the first device die and the first through-via therein;and a second encapsulating material encapsulating the second device dieand the second through-via therein.
 11. The package of claim 10, whereina backside of the first device die and a front side of the second devicedie faces the first plurality of dielectric layers.
 12. The package ofclaim 10 further comprising: a second plurality of dielectric layersunderlying and in contact with the first device die and the firstthrough-via; and a plurality of redistribution lines in the secondplurality of dielectric layers, wherein the plurality of redistributionlines is electrically coupled to the first device die and the firstthrough-via.
 13. The package of claim 10 further comprising: a thirdplurality of dielectric layers overlying and in contact with the seconddevice die and the second encapsulating material; an additionalplurality of redistribution lines in the third plurality of dielectriclayers, wherein the additional plurality of redistribution lines iselectrically coupled to the second through-via; a third device die overthe third plurality of dielectric layers; and a third encapsulatingmaterial encapsulating the third device die therein.
 14. The package ofclaim 13, wherein the third device die comprises a metal pillar, withthe metal pillar in contact with both a conductive feature in the thirdplurality of dielectric layers and a top dielectric layer in the thirdplurality of dielectric layers.
 15. The package of claim 10, wherein thesecond device die comprises: a semiconductor substrate, and athrough-semiconductor via penetrating through the semiconductorsubstrate, wherein the through-semiconductor via is electricallydecoupled from all active and passive devices in the second device die.16. A package comprising: a first plurality of redistribution lines; afirst plurality of dielectric layers, with the first plurality ofredistribution lines being in the first plurality of dielectric layers;a first device die underlying the first plurality of dielectric layers;an adhesive film joining the first device die to the first plurality ofdielectric layers; a second device die comprising metal pads, whereinthe metal bonds are bonded to the first plurality of redistributionlines; a first encapsulating material encapsulating the first device dietherein; and a second encapsulating material encapsulating the seconddevice die therein.
 17. The package of claim 16, wherein the adhesivefilm is encapsulated in the first encapsulating material.
 18. Thepackage of claim 16, wherein a top surface of the adhesive film and atop surface of the first encapsulating material are substantiallycoplanar.
 19. The package of claim 16, wherein a top surface of theadhesive film and a top surface of the first encapsulating material arein contact with a same dielectric layer in the first plurality ofdielectric layers.
 20. The package of claim 16 further comprising: anadditional dielectric layer over the second encapsulating material; andan underfill overlying and in contact with the additional dielectriclayer.